In establishing a subbottom facility in competent rock, there appear 
to be two distinct types of excavation, probably with separate solutions. In 
Figure 58a, a large-diameter vertical hole is shown in the bottom rock. Entry 
is made through the overlying soft material by casing and pumping as discussed 
earlier in ‘‘Casing Through Sediments.’ The vertical hole would be bored by 
a specialized mole initially guided by the steel casing through silt. The mole 
would receive its power from the surface and would make initial penetration 
into the rock under its own weight. Once into the rock, it could react hydrau- 
lically against the rock walls to augment the weight in downward thrust to 
obtain maximum drilling rate and optimum tool life. The machine parts 
would be exposed to full ocean pressure. As seen now in terms of conven- 
tional tunneling, the most difficult aspect of this operation is replacing worn 
cutters. The best (and probably most expensive) cutters available would be 
used to avoid retrieval of the mole for cutter replacement. If practicable, 
boring of the desired vertical hole without cutter replacement should be 
planned, even if this should lead to very high cost cutters being used. The 
spoil would be pumped to the surrounding ocean bed. 
The second mode of operation to be used in the lateral excavation 
from the vertical shaft (Figure 58b) might be done with a smaller specialized 
drilling machine (mole), or specialized fragmenting and mucking techniques 
capable of excavating large noncircular caverns of perhaps very complicated 
plan. |n above-sea-level operations where air is plentiful and integrity of the 
surrounding rock is of less importance, this type of excavation is most fre- 
quently and inexpensively done by drilling and blasting. An interisland tunnel 
between the main islands of Japan is being built by drilling and blasting, but 
with damaging effects. Drilling contractors from this operation attended a 
major international symposium on excavation in hopes of hearing of an 
excavation method which would cure their troubles (National Research 
Council, 1968, verbal comments at meeting). Blasting is a drastic process, 
only partially controllable, that causes considerable overexcavation and 
breaking of the surrounding rock, leading to excessive entry of water through 
the cracks. To seal these, all the overbreak must be replaced with a concrete 
or other waterproof liner. In the deep ocean, an additional problem, hydro- 
static head on the excavation, will arise once the entry shaft (Figure 58a) has 
been sealed (Figure 58b) and the cavity dewatered. At great depths (6,000 
to 12,000 feet), the requirements for continuous rock integrity will be so 
great that drilling and blasting, even if technically practical, will not be 
permissible. Rather, a more precise and gentle method of local disintegra- 
tion of the rock will be needed. Many methods have been proposed (Maurer, 
1968), but not all have been investigated. These methods and some variations 
are discussed later in this chapter. 
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